The rapid growth in the semiconductor electronics industry over a wide range of applications, has led to some associated packaging, handling, and operating considerations for electronic devices. One consideration for many electronic devices is that static electricity be controlled in some fashion. For example, because electronic components are becoming smaller and smaller, and operating on smaller power and current levels, the generation of static electricity, which would be considered no more than a nuisance in other circumstances (e.g., getting a shock after walking on a synthetic carpet), can become a significant problem with electronic components and devices. For example, static electricity can affect the memory or performance characteristics of small integrated circuits or "chips." From a physical standpoint, static electricity, which has the tendency to cause small objects to be attracted to one another, can cause handling problems when the small delicate parts used in many electronic applications are being physically handled. Static electricity can affect memory or programming aspects of a chip either temporarily or permanently and can interfere with the operation of electronic devices, or their memory components, in a number of ways including loss of data on magnetic media, equipment damage, and even fire hazards.
Accordingly, control of static electricity, whether by shielding, grounding, or some other technique is a necessary consideration in the manufacture and use of electronic devices.
One application for which static electricity must be controlled is the handling during manufacture of semiconductor chips as they are being assembled into larger circuits and electronic devices. One such handling technique is set forth in copending application Ser. No. 08/252,177, U.S. Pat. No. 5,447,784, filed Jun. 2, 1994, for "STATIC DISSIPATIVE COVER TAPE" which is assigned to the assignee of the present invention. As set forth therein, in order to be conveniently packaged for automated or robotic handling and assembly, integrated circuits are often packaged in longitudinal strip packages which consist of molded pockets covered with an adhesive tape that completes the package. The adhesive tape is removed and the pocketed carrier advanced by mechanical devices. Because the chips are sensitive to static electricity, the packaging tape must likewise be either antistatic or static dissipating in character.
As generally used in this field, the terms "antistatic" and "static dissipating" both refer to the same property: conductivity. The term "static dissipating" generally refers to a higher conductivity than does the term "antistatic." For example, antistatic is often used to characterize resistivities of 10.sup.9 to 10.sup.14 ohms per square, while static dissipative is used to characterize resistivities of 10.sup.5 to 10.sup.9 ohms per square. It will be understood that these terms are thus used descriptively rather than in any absolute or unreasonably limiting sense.
One way to make a cover tape--or indeed any similar surface--static dissipating or antistatic in character is to coat it with a composition that will both adhere to the tape and provide the necessary conductivity properties. Conductive coatings can be formed of a number of different materials, all of which have various advantages and disadvantages. For example, a thin metal coating will be conductive and therefore antistatic or static dissipating, but metal coatings can be expensive and difficult to apply. Furthermore, when metals are applied in amounts sufficient to provide the necessary conductivity, they may make the surface opaque, or otherwise change its appearance, a factor which is undesirable in many circumstances.
Other conductive materials such as carbon black are appropriate in different circumstances, but as with metals, carbon black (as indicated by its common name) is generally unsuitable for antistatic or static dissipating applications where the color or transparency of a given substrate are of importance to the finished product.
Additional choices for conductive materials include monoacyl glycerides, monoalkyl phosphates, and various metallocenes. Some of these exhibit solubility problems, however, or tend to decompose at lower temperatures than are generally convenient for use in certain circumstances.
Yet another category of antistatic compositions includes the quaternary ammonium salts; i.e., organic nitrogen compounds that include a central nitrogen atom joined to four organic groups (the cation) and a negatively charged acid radical (the anion). Lewis, HAWLEY'S CONDENSED CHEMICAL DICTIONARY, 12th Ed. (1993) p. 986. These salt compositions are well-known, predictable in their antistatic characteristics, and soluble in water and in certain organic solvents. They tend, however, to be quite sensitive to relative humidity and in particular their conductive properties tend to fade or disappear at lower relative humidities; i.e., relative humidities of about 20% or less. Therefore, they are either undesirable or simply unusable for particular applications.
Therefore, there exists the need for compositions that can produce antistatic or static dissipating properties on substrates that can be easily coated on those substrates, will adhere to them properly, and will provide antistatic properties even under varying conditions of relative humidity.